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Submitted on 1 Jan 1979
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EFFECT OF POLARITY ON THE IMPULSE BREAKDOWN OF SF6
E. Antonios Vlastós
To cite this version:
E. Antonios Vlastós. EFFECT OF POLARITY ON THE IMPULSE BREAKDOWN OF SF6. Jour-
nal de Physique Colloques, 1979, 40 (C7), pp.C7-253-C7-254. �10.1051/jphyscol:19797124�. �jpa-
00219096�
JOURNAL DE PHYSIQUE CoZZoque C7, suppt6ment au n07, Tome 40, k ~ ~ i l Z e 6 1979, page ~ 7 - 253
EFFECT OF POLARITY ON THE I M W S E BREAKDOWN OF
E. Antonios ~last6s.
l'he Royal I n s t i t u t e of TeehnoZogy, S-100 44 S t o ~ Abstract: The e f f e c t of p o l a r i t y on t h e impulse
breakdown of SF6 and t h e i n f l u e n c e which a t h i n d i e l e c t r i c coating has on t h e impulse breakdown f o r both p o l a r i t i e s wbs s t u d i e d i n t h e pressure range of 1-7 bara i n a c o a x i a l c y l i n d e r e l e c t r o d e confi- guration.
I n t r o d u c t i o n : The thought o f e s t a b l i s h i n g a s i m i - l a r i t y law f o r t h e breakdown o f gases dates back t o t h e end o f t h e previous century. Paschen i n t h i s t h e s i s , which appeared 1598, was t h e f i r s t t o estab- l i s h a r e l a t i o n between t h e breakdown v o l t a g e o f a gas and t h e product o f pressure times t h e e l e c t r o d e s e p a r a t i o n f o r p l a n e p a r a l l e l electrodes. The s c i e n - t i f i c s c e p t i c i s m about t h i s r e l a t i o n , t h e so c a l l e d Paschen's law, s t a r t e d t o grow when experimental r e s u l t s from measurements w i t h d i f f e r e n t e l e c t r o d e m a t e r i a l s , e l e c t r o d e s p a c i n s and gases become a v a i l a b l e . Soon i t became c?ear t h a t t h e breakdown v o l t a g e of a gas, as determined experimentally, i s n o t a monotonous s i n v l e valued f u n c t i o n o f t h e pro- d u c t pressure p times t h e e l e c t r o d e s e p a r a t i o n d b u t shows a minimum. F u r t h e r more i t was found t h a t t h e r e l a t i o n between breakdown v o l t a g e and t h e pro- d u c t pd, a t values o f pd, which a r e e i t h e r l o w e r o r h i g h e r than t h a t corresponding t o Paschen's mi- nimum, deviates from t h e l i n e a r one and t h a t t h e d e v i a t i o n appears e a r l i e r f o r small e l ectrode sepa- r a t i o n s d. A l l these w e l l known f a c t s a r e summarized here t o emphasize t h a t from t l e beginning i t was a l - ready e s t a b l i s h e d t h a t t h e outcome from t r i a l s made t o determine t h e breakdown v o l t a g e o f a gas does n o t o n l y depend on t h e composition and i m p u r i t i e s o f t h e gas b u t a l s o depends on t h e electrodes, on t h e i r c o n s t i t u t i o n , shape and s u r f a c e c o n d i t i o n s (such as roughness and oxide l a y e r s ) . The main reason f o r t h e i n f l u e n c e o f t h e e l e c t r o d e s on t h e breakdown v o l t a g e was accepted t o be t h e e l e c t r o n emission from t h e e l e c t r o d e s u r f a c e which appears a t e l e v a t e d macro- scopic e l e c t r i c a l f i e l d s .
I n s l i g h t l y asymmetrical e l e c t r i c f i e l d s , such as t h e f i e 1 d produced between two c o a x i a l c y l i n d e r s , t h e r e i s experimental evidence t h a t n o t o n l y t h e e l e c t r o d e shape b u t a l s o t h e e l e c t r o d e s u r f a c e con- d i t i o n s a r e c r u c i a l f o r t h e outcome o f t h e measure- ments e s p e c i a l l y when t h e p o l a r i t y o f the e l e c t r o d e w i t h t h e s m a l l e r r a d i u s i s negative. Moreover, f o r e l e c t r o n e g a t i v e gases, where t h e breakdown v o l t a g e i s h i g h e r than t h a t f o r non-electron-attaching ga- ses t h e d e v i a t i o n s from t h e s i m i l a r i t y o r Paschen's law were found t o be l a r g e r and t o appear a t pres- sures which a r e lower than those corresponding t o n o n - e l e c t r o n - a t t a c h i n g gases. The appl i c a t i o n o f p r e s s u r i z e d e l e c t r o n e g a t i v e gases f o r t h e i n s u l a - t i o n of h i g h v o l t a g e swi tchgear equipment necessi- t a t e d an i n t e n s i v e research on t h e breakdown v o l t a g e o f e l e c t r o n e g a t i ve' gases and especi a1 l y o f SF6 which besides i t s f a v o r a b l e d i e 1 e c t r i c p r o p e r t i e s a1 so processes good a r c quenching p r o p e r t i e s . CIGRE's study committee 15 on i n s u l a t i n g m a t e r i a l s has f r c m a l r ~ a d y p u b l i s h e d work prepared a composite
Paschen's curve f o r SF6 and t h e d e v i a t i o n s from t h e Paschen's curve a t d i f f e r e n t e l e c t r o d e spa- c i n g s f o r e s s e n t i a l l y u n i f o r m f i e l d s [I
I .
The de- v i a t i o n s from t h e Paschen's curve appear a t a mean e l e c t r i c a l f i e l d o f about 20 kV/mm.Results: I n o r d e r t o study t h e e f f e c t o f p o l a r i t y
impulse
breakdown o f SF6 and t h e i n f l u e n c e which a t h i n d i e l e c t r i c c o a t i n g has on t h e impulse breakdown f o r b o t h p o l a r i t i e s extensive measure- ments were performed w i t h c o a x i a l c y l i n d r i c a l e l e c t r o d e s w i t h diameters o f 70 and 30 mm respec- t i v e l y and a l e n g t h over which the f i e l d was un- d i s t o r t e d of 250 mm. F i g u r e 1 shows t h e 1 % p o s i - t i v e l i g h t n i n g impulse (1.2/50 p) breakdown v o l - tage o f SF6 w i t h bare p o l i s h e d pure aluminium and w i t h p o l i s h e d and l a t e r anodized pure aluminium i n n e r e l e c t r o d e as a f u n c t i o n o f t h e gas pressure.For comparison t h e 1 % breakdown v o l t a l e s o f a i r w i t h anodized e l e c t r o d e s a r e a l s o given. F i q u r e 2 shows t h e 1 % n e o a t i v e 1 i ~ b t n i n n i r w l s e !1.2/50
us)
breakdown v o l t a g e o f SF6 and a i r w i t h t h e same type o f e l e c t r o d e s as those i n f i g u r e 1. The 1 % break- down probabi 1 i t i e s o f bare and anodized e l e c t r o d e s were d e r i v e d by a p p l y i n g a hundred impulses on a new e l e c t r o d e p a i r a t each v o l t a g e l e v e l and by i n - c r e a s i n g t h e v o l t a g e l e v e l s by small steps u n t i l t h e f i r s t breakdowns occured. T h i s measuring pro- cedure seemed t o be t h e most s u i t a b l e s i n c e i t r e - duces t h e i n f l u e n c e o f previous breakdowns on t h e outcome o f r e s u l t s . The p r o f i l e curve o f t h e po- l i s h e d e l e c t r o d e s showed t h a t t h e d i f f e r e n c e b e t - ween t h e c r e s t and bottom l i n e s o f t h e p r o f i l e curve was about 2-4m.
The o x i d e l a y e r thickness o f t h e anodized aluminium e l e c t r o d e s was about 10m
and t h e d i f f e r e n c e between the c r e s t and bottom l i n e s o f t h e p r o f i l e c u r v e o f the anodized e l e c t r o d e s was about 1m.
The r e s u l t s presented i n f i g u r e s 1 and 2 a r e b r i e f - l y summarized as f o l l o w s : (a) The 1 % impulse breakdown v o l t a g e a t 1 bara i s w i t h i n t h e accuracy o f t h e experiments almost equal both f o r bare and covered e l e c t r o d e s and f o r t h e two p o l a r i t i e s .
(b) A t h i g h e r pressure t h e d i f f e r e n c e between t h e 1 % impulse breakdown v o l t a g e f o r p o s i t i v e and ne- g a t i v e impulses increases. ( c ) The improvement due t o t h e a n o d i z a t i o n o f t h e e l e c t r o d e s i s l a r g e r w i t h n e g a t i v e p o l a r i t y making t h e 1 % n e g a t i v e im- p u l s e breakdown v o l t a g e almost equal t o t h a t o f bare e l e c t r o d e s w i t h p o s i t i v e p o l a r i t y .
A p o s s i b l e e x p l a n a t i o n o f these r e s u l t s i s t h a t t h e g r e a t d e v i a t i o n s between p o s i t i v e and n e g a t i v e im- p u l s e breakdown voltages i n gases such as SF and the d e v i a t i o n s from Paschen's law i s i n one Rand due t o t h e e l e c t r o n emission from t h e s u r f a c e and especi a1 l y t o t h e e l e c t r o n v e l o c i t y d i s t r i b u t i o n and on the o t h e r hand t o t h e s t r o n g r e d u c t i o n o f t h e e l e c t r o n attachement cross s e c t i o n of SF6 a t r e l a t i v e l y low e l e c t r o n energies
121.
By p o l i s h i n g and by anodizing t h e e l e c t r o d e s the e l e c t r o n energyArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797124
Aluminium electrodes Aluminium electrodes
Fig. 1 . Dependence on gas p r e s s u r e o f t h e 1 % F i g . 2. Dependence on gas p r e s s u r e o f t h e 1 %
p o s i t i v e ' l i g h t n i n g impulse ( 1.2/50 u s ) n e g a t i v e l i g h t n i n g impulse (1.2/50 u s ) breakdown v o l t a g e o f SF6 and a i r w i t h breakdown v o l t a g e o f SF6 and a i r w i t h b a r e p o l i s h e d pure aluminium and f o r ano- b a r e p o l i s h e d p u r e aluminium and f o r ano- d i z e d p l i s h e d pure aluminium i n n e r e l e c - d i z e d p o l i s h e d pure aluminium i n n e r e l e c -
t r o d e . t r o d e
